Many of today's portable appliances incorporate rechargeable batteries. Example appliances include electrical toothbrushes, electric shavers, beard trimmers, and/or depilation devices. The appliances may include other chargeable appliances also, such as cellular telephones or laptop computers.
In some of these devices, the batteries are recharged via inductive coupling rather than a direct electrical connection. Examples include the Philips Sonicare™ toothbrush, the Panasonic Digital Cordless Phone Solution KX-PH15AL and the Panasonic men's shavers ES70/40 series.
Each of these devices typically has an adaptor or charger which takes power from mains electricity, a car cigarette lighter or other sources of electrical power and converts it into a form suitable for charging the batteries. There are a number of problems associated with conventional means of powering or charging these appliances.
Both the characteristics of the batteries within each appliance and the means of connecting to them vary considerably from manufacturer to manufacturer, and from device to device. Therefore users who own several such appliances must also own several different adaptors. If users are going away on travel, they will have to bring their collection of chargers if they expect to use their devices during this time.
Chargers, whether using direct electrical connection or inductive coupling, often require users to plug a small connector into the appliance or to place the device with accurate alignment into a stand causing inconvenience.
In addition, most adaptors and chargers have to be plugged into mains sockets and hence if several are used together, they take up space in plug strips and create a messy and confusing tangle of wires.
Besides the above problems, there are additional practical problems associated with chargers having an open electrical contact. For example, such chargers cannot be used in wet environments due to the possibility of corroding or shorting out the contacts and also they cannot be used in flammable gaseous environments due to the possibility of creating electrical sparks.
Chargers which use inductive charging remove the need to have open electrical contacts hence allowing the adaptor and device to be sealed and used in wet environments. For example the electric toothbrush, the shaver and the depilation device as mentioned above are designed to be used in a bathroom. However such chargers still suffer from all other problems as described above. For example, the devices still need to be placed accurately into a charger such that the device and the charger are in a predefined relative position. The adaptors are still only designed specifically for a certain make and model of device and are still only capable of charging one device at a time. As a result, users still need to possess and manage a collection of different adaptors.
U.S. Pat. No. 7,248,017-B2 in the name of Splashpower Limited (UK) recognizes the above problems and describes a number of solutions to overcome the limitations of inductive power transfer systems which require that the appliances be axially aligned with the charger.
A relatively simple solution is to use an inductive power transfer system whereby the primary unit is capable of emitting an electromagnetic field over a large area. Users can simply place one or more devices to be recharged within range of the primary unit, with no requirement to place them accurately. For example this primary unit may consist of a primary coil encircling a large area. When a current flows through the primary coil, an electromagnetic field extending over a large area is created and devices having a secondary pickup coil can be placed anywhere within this area. This method suffers from a number of drawbacks. Firstly, the intensity of electromagnetic emissions is governed by regulatory limits. Consequently this method can only transfer power at a limited rate. Secondly, as the pick-up coil will by definition encircle a much smaller area than the primary coil, the pick-up coil will only enclose a correspondingly small part of the magnetic field that is generated by the primary coil. This so called magnetic coupling is dependent on for instance the distance, cross section and orientation of the pick-up coil in relation to the primary coil.
The magnetic coupling coefficient k21 of the secondary coil to the primary coil is expressed as the ratio of the flux Φ2 enclosed by the secondary coil and the flux Φ1 enclosed by the primary coil:k21=Φ2/Φ1 
The lower the coupling coefficient, the weaker the magnetic coupling, and the higher the current in the primary coil must be to transfer the same amount of energy. Increasing the current in the primary coil increases energy loss due to electrical resistance. To render the energy transfer efficient and to limit energy losses, it is desirable to optimize the magnetic coupling between the primary coil and the pick-up coil.
To optimize magnetic coupling, one might suggest using an array of primary coils whereby only the coils needed are activated. This method is described in a paper published in the Journal of the Magnetics Society of Japan titled “Coil Shape in a Desk-type Contactless Power Station System” (29 Nov. 2001). In an embodiment of the multiple-coil concept, a sensing mechanism senses the relative location of the secondary device relative to the primary unit. A control system then activates the appropriate coils to deliver power to the secondary device in a localized fashion. Although this method provides a solution to the problems previously listed, it does so in a complicated and costly way. The degree to which the primary field can be localized is limited by the number of primary coils and hence the number of driving circuits used. The cost associated with a multiple-coil system would severely limit the commercial applications of this concept. Non-uniform field distribution is also a drawback. When all the coils are activated in the primary unit, they sum to an equivalent of a large coil, the magnetic field distribution of which is seen to exhibit a minimum at the centre of the coil.
U.S. Pat. No. 7,248,017-B2 discloses a rechargeable battery which comprises a thin sheet of magnetic core material and a pick-up coil wound around the thin sheet. The pick-up coil is adapted to receive power inductively from an external unit. The external unit generates an electromagnetic field at or over a field generating surface. To charge the battery, the coil is placed in proximity to the field generating surface, such that a longitudinal axis of the sheet and a central axis of the coil each extend parallel to the surface. The external unit comprises a number of conductors for generating a rotating magnetic dipole along the field generating surface, such that the coil can receive power regardless of its rotational orientation. Unlike the multiple coil design, this solution requires a simpler control system and fewer components.
Regardless of its advantages, the solution of U.S. Pat. No. 7,248,017-B2 is unfavorable due to weak coupling between the field generating surface and the pick-up coil, and the resulting power losses and low efficiency. The conductors of the external unit encircle an area that is much larger than the area enclosed by the pick-up coil. To increase the magnetic coupling, U.S. Pat. No. 7,248,017-B2 teaches to include high permeability magnetic material in the secondary device to increase the induced flux by offering a low reluctance path. Due to the weak magnetic coupling, a relatively high current is provided to the conductors. To compensate the magnetic flux generated by this current, a layer of magnetic material is present beneath the charging area to provide a return path for the flux.